In telecommunications, it is often desired to convert an analog input signal into a binary digital output signal, in order to perform more economically such signal processing operations as signal amplification or regeneration along the transmission path. However, present-day analog-to-digital (A/D) converters tend to be rather expensive, owing to their hybrid structures with precise required tolerances, that is, precisely manufactured individual elements not in the form of compact integrated circuits.
In U.S. Pat. No. 3,958,210, issued to P. A. Levine on May 18, 1976, an electronic system for analog-to-digital conversion was disclosed including a semiconductor charge coupled device, utilizing the properties of semiconductor surface potential wells for charge storage and transfer in response to voltages applied to electrodes overlying the wells. However, the semiconductor charge coupled device portion of that system produced only a digital counting representation (unitary based number system) of the input analog signal, and the system required complex logic circuitry to convert this digital counting representation into the ultimately desired representation in the binary number system. In other words, an analog input representing the number n was converted by the charge coupled device portion into a "unitary" sequence purely of n "ones" (1,1,1, . . .1,1,1) according to the unitary number system, rather than directly into the desired binary sequence of "ones" and "zeros" according to the binary number system. Complex logic circuitry was thus required for subsequent conversion of the "unitary" sequence into a corresponding binary digital sequence such as (1,0,1, . . . 0,1,1) representing n=1.times.2.sup.i + 0.times.2.sup.i-1 + 1.times.2.sup.i-2 + . . . + 0.times.2.sup.2 + 1.times.2.sup.1 + 1.times.2.sup.0, where i is selected such that 1.times.2.sup.i is the "most significant bit" in the number n.
Accordingly, it would be desirable to have semiconductor apparatus for converting an analog signal directly into its representation in the binary number system.